The present invention relates to a switched multipath filter network which is defined herein as a filter network with a plurality of low-pass filters operating in parallel under the control of cyclically operated switches such that the network has the characteristics of a band-pass filter with its pass-band centered on a frequency which is determined by the frequency of the switch operating cycle.
In some applications of such filter networks, it is desirable to be able to select different pass-bands. There is no difficulty in changing the pass-band centre frequency; this is done by changing the frequency of the switch operating cycle. However, it may also be desirable to be able to change the bandwidth, e.g., in use in a two-tone frequency shift keyed system, and it is with this problem that the present invention is concerned. Before explaining the invention, the general background thereto will be briefly outlined.
It is difficult to design passive or active band-pass filters which have both a stable centre frequency and a high Q, such as Q=50. It is known, however, that a band-pass filter may be formed by operating a plurality of low-pass filters in parallel, each filter being preceded by and followed by modulators which are driven by modulating signals at the desired centre frequency and such that the input signal is converted down to the difference frequency, low-pass filtered and then converted back to the original frequency. The modulating signals form an N-phase system (N being the number of filter paths), and the outputs of these paths are additive and such that the output is filtered in accordance with a pass-band extending from f.sub.s -f.sub.c to f.sub.s +f.sub.c, where f.sub.s is the modulating signal frequency and f.sub.c is the cut-off frequency of each low-pass filter.
In practice, an arrangement of this nature is rarely employed because of its expense, but it is known that the modulating signal need not be a sinusoid at f.sub.s but may be a square wave at f.sub.s and this leads to simplified arrangements, which are employed, in which the modulators are replaced by switches controlled by the square waves of different phases, which may now be better referred to as sampling signals; each filter performs a sampling function. There may be, by analogy with the network having modulators, series switches preceding and following each low-pass filter, but it is known that other arrangements of switches are feasible, e.g., switches in the shunt arms of the low-pass filters. In any event, the use of the switches does more than replace the function of the modulators. The sampling action causes each filter to be operative for a limited duty cycle which has the same effect as if some component values were modified and the network typically has a main pass-band extending from f.sub.s -f.sub.c /N to f.sub.s +f.sub.c /N. The bandwidth has been reduced by a factor of N from 2f.sub.c to 2f.sub.c /N and this assists in achieving a narrow pass-band, which is centred on the frequency f.sub.s which can be accurately determined by the square-wave generator which provides the sampling signals. The factor of N presupposes that the duty cycle is 1/N, that is to say, the switches are operated without either overlaps or gaps. As well as the main pass-band, there is a series of harmonic bands nf.sub.s .+-.f.sub.c /N where n is the order of the harmonic. The values of n which are present depends on the number of paths. With N=4, for example, the second and third harmonics (n=2 and n=3) are present, but the fourth harmonic is absent. Regardless of which harmonic bands are present, it is a simple matter to reject them all by a low-pass filter with a cut-off frequency in the stop band between the main pass-band and the first harmonic band which is present.